The important physiological role of the peptides released from fibrinogen is being investigated. These peptides produce strong hemostasis in rat tail arterioles and induce hypertension after intravenous injection into the rat. The fibrinopeptides also act on the contractile proteins by increasing both the rate of superprecipitation and the hydrolysis of ATP. The effect of structural modification upon the thrombin-induced conversion of fibrinogen to fibrin is being studied as a model for better understanding of the molecular aspects of the action of thrombin on fibrinogen.